1. Field of the Invention
The invention is concerned with catalysts and catalysis. While the invention is of particular interest with respect to conversion of polluting NO.sub.x compounds to non-polluting form, it is concerned also with catalysis of a more general class of reactions including oxidation as well as reduction.
2. Description of the Prior Art
There is rapidly growing concern with the ecological effects of effluents produced by a variety of processes. A source of pollution of particular concern to many is exhaust produced by light vehicles powered by internal combustion engines. Pollutants characteristically contained in such exhaust are classified either as incompletely combusted fuel--e.g., CO and unburned hydrocarbon--or oxidized material ancillary to the combustion process--e.g., oxides of nitrogen sometimes designated NO.sub.x.
Growing concern with pollution resulting from automobile exhaust has prompted legislation and a variety of research and development programs in many countries. In the United States, for example, legislation provides for reduced CO, hydrocarbons and NO.sub.x emission in 1973 model year automobiles; further reduced CO and hydrocarbon emission in 1975 model year automobiles; and further decreased NO.sub.x in the 1976 automobile model year.
Several approaches have been taken to minimize content of incompletely combusted emission products. These have included after burners, electronically controllable fuel injection (to optimize air-fuel mixture under all driving conditions) stratified charge, (providing for ignition in an enriched mixture with progression of the burning front into a lean mixture. Ignition is not easily initiated in a mixture sufficiently lean to assure substantially complete combustion), and catalytic converters.
A complete solution to the problem of incompletely combusted emission pollutants is clearly available. Several vehicles, although generally of unconventional engine design, meet the standards. Catalysts also are promising, and substantially complete conversion of CO and hydrocarbons to CO.sub.2 and H.sub.2 O has been shown to occur under a variety of conditions. At this time, it is likely that a greater development effort is being directed to platinum-containing catalysts than any others despite a variety of problems. These include lead poisoning due to tetraethyl lead, included as an anti-knock agent in gasoline; the high cost of such precious metal-containing catalysts and an apparently limited supply. Other catalysts, however, may be developed to satisfactorily meet this need. These include perovskites of the class exemplified by (La,Pb)MnO.sub.3 (177 Science 353 (1972)).
Nitrogen oxides, in the view of many, are not as easily disposed of. This class of pollutants which occurs as a by-product in the operation of high efficiency, high combustion ratio engines has not as easily yielded to catalysis. It has been the general consensus that NO.sub.x is not catalytically decomposed to yield N.sub.2 and O.sub.2 --this would be the ideal solution. The catalytic conversion of NO.sub.x to N.sub.2 in the presence of a reducing ingredient, such as CO, has been carried out with some degree of success with metallic catalysts but work to date has generally been of limited promise by reason either of catalyst instability or poor low temperature activity. At lower operating temperatures, catalysts have generally been ineffective, or else where hydrogen is present in the form of H.sub.2 or H.sub.2 0 have resulted in the production of NH.sub.3. Ammonia gas is, itself, a pollutant, and further, if passed through a catalytic chamber designed to oxidize CO, generally undergoes reoxidation to NO. Production of NH.sub.3 at higher temperatures if it occurs is not so significant, since this compound is quite unstable above about 500.degree. C. Where decomposition does not occur spontaneously, it may be catalyzed by any of a number of NH.sub.3 decomposers.
By reason of the above, the general approach to the removal of NO.sub.x has assumed a different direction. In the United States, for example, 1973 model year automobiles are provided with means for recirculation of a fraction of the exhaust gas to the engine. Recirculation of a fraction of about 10 percent of the exhaust results in substantial lowering of NO.sub.x emission. This is due, in part, to the shift in equilibrium resulting from the concentrating of NO.sub.x but, more significantly, to the displacement of oxygen thereby resulting in a cooler burning front in the engine cylinder. Recirculation, while accomplishing its objective--reduction of NO.sub.x emission--has the alarming effect of significantly decreasing engine efficiency. It has been estimated that increased fuel consumption due to exhaust recirculation is at a minimum of 10 percent. It is expected that this expedient will result also in increased maintenance problems, and operational difficulties have already been experienced.
Preliminary investigation of a class of perovskite catalysts has shown significant activity in the catalytic removal of NO.sub.x as well as of CO and unburned hydrocarbons over temperature ranges including those which occur shortly after automobile start-up. The prototype material useful for both types of reactions may be represented by the atom formula RE.sub.1-x M.sub.x MnO.sub.3 in which RE is La, Pr, or Nd; and M is Pb, Sr, Ca, or Ba. See 177, Science 353 (1972) and 180 Science, 62 (1973). Some enhancement in the activity of such catalysts designed for NO.sub.x removal has resulted from acid etching (180 Science, supra.) and in partial substitution of manganese by Ru and/or Ni. See US Application Ser. No. 364,260 filed May 29, 1973. Catalysts of this class are of both scientific and practical interest in that a significant part of the nitrogen-containing reaction product is N.sub.2 O. In general, production of this material has been significant over a temperature range of up to about 350.degree. C.